Steel cord with waved elements

ABSTRACT

A steel structure adapted for the reinforcement of elastomeric members has steel elements containing a plurality of steel filaments at least one of which filaments is provided with first and second crimps. The first crimp lies in a plane that is substantially different from the plane of the second crimp. Application of the both crimps can be carried out efficiently using two pairs of toothed wheels which are not externally driven. This arrangement renders it possible to obtain steel structures with an increased penetration of rubber or with an increased elongation at break.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/555,045, filed May 24, 2000, now U.S. Pat. No. 6,311,466.

FIELD OF THE INVENTION

The present invention relates to a steel structure adapted for thereinforcement of elastomers such as rubber conveyor belts, rubber tyres,rubber hoses, rubber timing belts or timings in polyurethane. The steelreinforcement comprises one or more steel filaments.

The present invention also relates to a method of treating a steelfilament so that the steel filament receives a spatial wave form.

BACKGROUND OF THE INVENTION

Such steel structures are widely known in the art. Recent prior artdocuments have disclosed a tendency towards steel structures where thesteel filaments present one or another type of waviness, i.e. where, inaddition to the plastic deformation as a consequence of the possibletwisting of the steel filaments, the steel filaments have anotherplastic deformation. This additional and other plastic deformation isconveniently a consequence of a preforming operation, and results in awavy pattern on the steel filament.

In this way U.S. Pat. No. 5,020,312 (Kokoku—priority 1989) and U.S. Pat.No. 5,111,649 (Kokoku) disclose steel cord structures consisting ofthree to five steel filaments. At least one steel filament is providedwith a so-called ‘crimp’: this is a zigzagged form with relatively sharpangles, the sharpness depending upon the formation tools. The crimp is aplanar wave form and is formed by means of two toothed wheels. The holescreated at the level of the angles promote penetration of elastomer intothe steel cord structure.

Another wave form has been disclosed in EP-A-0 462 716 (Tokusen—priority1990). According to this document, the steel cords have three totwenty-seven steel filaments, 25% to 67% of which have a particularhelix or helicoidal form. The plastical helix deformation is carried outby means of rotating preforming pins. The purpose is to promotepenetration of the elastomer into the steel cord structure withoutincreasing the so-called part load elongation (PLE, for definition seebelow). These steel cords are marketed under the name SPACY® cord. Animportant drawback of this cord is that its manufacture isenergy-consuming or inefficient or both. Indeed, if the pitch of thehelix is taken smaller than the twist pitch, then the rotation speed ofthe preforming pins must be more than twice as high as the rotationspeed of a down-stream double-twister.

Still another wave form has been disclosed in WO-A-95/16816(Bekaert—priority 1993). According to this document, the steel structurecomprises steel filaments and at least one steel filament has beenpolygonally preformed. This is a spatial wave form and is the result ofa preforming device with varying radii of curvature. The steelstructures are marketed under the name BETRUO®.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide still another waveform to steel filaments of steel structures.

It is another object of the present invention to provide a wave form tosteel filaments where the wave form combines advantages of existing waveforms.

It is still another object of the present invention to provide a waveform which can take a lot of specific forms depending upon the choice ofthe parameters of the wave form.

It is yet another object of the present invention to provide a waveform, the manufacture of which does not necessitate energy-consumingtools.

It is also an object of the present invention to provide a steelstructure with an oval transversal cross-section as a consequence of thewave form of some filaments, e.g. a core filament.

According to the invention, there is provided a steel structure adaptedfor the reinforcement of elastomers. The steel reinforcement comprisesone or more steel elements. At least one of these steel elements isprovided with a first crimp and a second crimp. The first crimp lies ina plane that is substantially different from the plane of the secondcrimp.

In this way a spatial wave form is obtained without using driven andenergy-consuming preforming tools.

Another advantage of this steel structure is that a lot of wave formsbecome possible. Indeed, the first crimp has a first crimp pitch and afirst crimp amplitude. The second crimp has a second crimp pitch and asecond crimp amplitude. This means already four design parameters whicheach can be varied, independently of each other over a certain range.

The first crimp pitch may be equal to or different from the second crimppitch. With equal crimp pitches circular or oval spatial helixes can beobtained. Different crimp pitches, however, lead to spatial formsdifferent from helixes.

The first crimp amplitude may be equal to or different from the secondcrimp amplitude. A different crimp amplitude enables to obtain a spatialform with an oval transversal cross-section on condition that thefilament which is provided with the first crimp and the second crimp isnot rotated around its own axis in the final steel structure.

Still another parameter which can be varied is the angle between the twoplanes. It is preferable, however, that the planes differ as much fromeach other as possible: so the best choice is a maximum difference ofabout 90°.

The steel element of the steel structure according to the invention canbe a steel filament, a bundle of non-twisted steel filaments or a steelstrand of twisted steel filaments. The steel structure according to theinvention may also be constituted by any combination hereof.

The steel structure may be an untwisted structure consisting of one ormore steel filaments lying parallel adjacent to each other and bound byeach other by means of another wrapping filament or by means of anadhesive that is compatible with the elastomer to be reinforced.

An alternative embodiment is that the steel filaments lie nearlyparallel adjacent to each other, which can be obtained by twisting themwith a very large twist pitch e.g. by passing them at a relatively highlinear speed through a twisting apparatus rotating at a convenient orrelatively low rotation speed.

The steel structure may also be a twisted structure with some or all ofthe composing filaments twisted in to a coherent structure.

At least one of the first crimp pitch and the second crimp pitch ispreferably smaller than the twist pitch of the steel filament providedwith the first and the second crimp.

Within the general group of twisted structures, a first application ofthe invention are n×1 steel cords, i.e. cords essentially consisting oftwo to five steel filaments.

In a first embodiment, some but not all of these filaments are providedwith the first and the second crimp in order to allow rubberpenetration. An example is a 4×0.28 cord with one or two filamentsprovided with the first and the second crimp. Such a cord is used in thebreaker plies of a tyre.

In a second embodiment, all of the filaments are provided with the firstand the second crimp in order to increase the elongation at break above5% or more.

An example is a 5×0.38 cord with the five filaments provided with thefirst and second crimp. An additional advantage is that the cord may betwisted with a relatively large twist pitch (14 mm to 20 mm) withoutdecreasing substantially the elongation at break. Another example are4×0.22 and 5×0.22 where all filaments are provided with the first andthe second crimp. These high elongation cords are suitable forreinforcing tyres of a motor cycle (lying at nearly 0° with respect tothe equatorial plane of a motor cycle tyre).

A second application of the invention are the so-called/+m (+n) steelcords comprising a core of/core steel filaments and a layer of m steelfilaments twisted around the core. Additionally, a second layer of nsteel filaments may be twisted around the first layer of m filaments.

One or more core steel filaments may be provided with the first and thesecond crimp in order to:

a) increase the penetration of the elastomer into the core;

and/or to

b) obtain an oval transversal cross-section of the core, and as aconsequence, an oval transversal cross-section of the whole cord; and/orto

c) prevent the core steel filaments from migrating out of the cord.

An example is a 1+6 constructior. where the single core filament isprovided with a first crimp and a second crimp in order to enable rubberpenetration and in order to increase the anchorage of the single corefilament in the cord, i.e. to prevent core migration.

The first crimp amplitude may be greater than the second crimp amplitudeso that an oval transversal cross-section is obtained.

Another example is a 3+8+13 construction, where the three core filamentsare provided with a first crimp and a second crimp in order to allowrubber penetration to the centre between the three core filaments.

A similar application is the replacement of the core filaments of thestrands in a 7×7 construction by a 2×1 or 3×1 element where the two orthe three filaments are provided with the first and the second crimp.

Still another example is replacing the well-known construction3×d+9×d+15×d by a 5×d₁+9d+15×d, where the filament diameter d1 of thecore filaments is smaller than the diameter d of the other filaments.The core filaments are provided with the first and the second crimp.Rubber penetration and elongation are increased and the stiffness isdecreased.

A third application of the invention is the so-called n×1 compact cordscomprising n steel filaments which have been twisted with each other inthe same twist sense and with the same twist pitch. An example is a3×0.365 |9×0.345 CC (CC=compact cord) where all the core filaments areprovided with the first and the second crimp in order to provide rubberpenetration and in order to prevent core migration.

Another example is a 12×0.38 CC where all twelve filaments are providedwith the first and the second crimp in order to obtain a highelongation. Such a cord can be used as the weft or warp element in awoven structure adapted to reinforce rubber conveyor belts.

A fourth application is the multi-strand steel cord, which is a steelcord comprising two or more strands and where each strand consists oftwo or more filaments. if such strands are twisted in the cord in thesame sense as the filaments are twisted in the strand (the so-calledLang's lay cords) a high elongation at break can be obtained. Acondition hereto is that relatively small twist pitches are used.

According to the invention, however, if some or preferably all filamentsare provided with the first crimp and the second crimp, then much largertwist pitches are possible without loosing any elongation at break, andthus cords are possible which can be manufactured in a more efficientway.

It is also possible, still according to the invention, to combine theexisting small twist pitches with a first and second crimp applied toall steel filaments. This allows to obtain a still higher elongation atbreak. The unavoidable loss in tensile strength and breaking load can becompensated by using an addition strand as core. The filaments of thiscore strand can also be provided with the first and second crimp.

A fifth application is a multi-strand steel cord, e.g. for use asreinforcement of conveyor belts, where the strands as a whole areprovided with a first crimp and a second crimp, e.g. in order to obtaina rubber penetration between the strands.

According to another aspect of the invention, there is provided a methodof giving to a steel filament a spatial wave form. The method comprisesthe following steps:

(a) applying a first crimp to said steel filament, said first crimplying in a first plane;

(b) applying a second crimp to said steel filament, said second is crimplying in a second plane substantially different from said first plane.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference tothe accompanying drawings wherein

FIG. 1 schematically illustrates how a first and a second crimp areprovided to a steel filament;

FIG. 2 shows the first crimp given to a steel filament;

FIG. 3 shows the second crimp given to a steel filament;

FIG. 4 shows a transversal cross-section of a 1×4 steel cord with twofilaments provided with the first and the second crimp;

FIG. 5 shows a transversal cross-section of a 1×5 steel cord with allfive filaments provided with the first and the second crimp;

FIG. 6 shows a transversal cross-section of a 1+6 steel cord with thecore filament provided with the first and the second crimp;

FIG. 7 shows a transversal cross-section of a 12×1 compact cord wherethe three central filaments are provided with the first and the secondcrimp;

FIG. 8 shows a transversal cross-section of a 4×2 multi-strand cordwhere all filaments are provided with the first and the second crimp;

FIG. 9 shows a load-elongation an invention 5×0.38 cord.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 schematically illustrates how a first crimp and a second crimpare provided to a steel filament 10.

The steel filament 10 is moved downstream towards a first pair oftoothed wheels 12. The axes of rotation of toothed wheels 12 lieparallel to the y-axis, and the first crimp given is a planar crimplying in plane xz.

The thus crimped filament 10 is further moved to a second pair oftoothed wheels 14. The axes of rotation of toothed wheels 14 lieparallel with the x-axis. The second crimp given by toothed wheels 14 isalso a planar crimp and lies in plane yz. Obviously the resulting wavegiven to the steel filament 10 is no longer planar but spatial.

Neither the first pair of toothed wheels 12 nor the second pair oftoothed wheels 14 need to be driven by external means. They are bothdriven and rotated by the passing steel filament 10.

It is important that the second pair of toothed wheels 14 is positionedas close as possible to the first pair of toothed wheels 12 in order toprevent the first crimp from tilting or rotating from plane xz to planeyz under influence of the second crimp. From a more general point ofview and in order to control the two crimps given to the filaments, the.bending moment, i.e. the moment necessary to give the two crimps, mustbe kept as small as possible. This can be done, e.g. by applying firstthe crimp with the smaller amplitude and only thereafter the crimp withthe greater amplitude.

Still from a more general point of view, the torsion moment, i.e. themoment necessary to rotate the filament, should be kept as high aspossible, since the rotating of the filament must be prevented during orbetween the two crimping operations. One way to keep the torsion momentas high as possible is the above-mentioned minimum distance between thetwo pairs of crimping wheels.

A third and following pairs of toothed wheels may be provided in otherplanes or in the same planes. In this way the spatial structure obtainedby the subsequent crimping operations may be optimised or varied to afurther degree.

FIG. 2 shows the first crimp lying in plane xz and FIG. 3 shows thesecond crimp lying in plane yz. The first crimp has a first crimpamplitude A₁, which is measured from top to top, with inclusion offilament diameter d. The first crimp has a first crimp pitch P_(c1),which is equal to the distance between two minima of the first crimp.

The second crimp has a second crimp amplitude A₂, which is measured fromtop to top, with inclusion of filament diameter d.

The second crimp has a second crimp pitch P_(c2), which is equal to thedistance between two minima of the second crimp. The spots 16 where thesecond crimp reaches its maxima are hatched in parallel with the axis ofthe steel filament 10, and the spots 18 where the second crimp reachesits minima are hatched vertically in FIG. 2.

The spots 20 where the first crimp reaches its maxima are hatched inparallel with the axis of the steel filament 10, and the spots 22 wherethe first crimp reaches its minima are hatched vertically in FIG. 3.Both the first crimp amplitude A₁ and the second crimp amplitude A₂maybe varied independently of each other. So A₁ may be equal to A₂ or maybe different from A₂. Both amplitudes may vary between a minimum valuewhich is slightly above value of the filament diameter (e.g. 1.05×d,which means almost no crimp), and a maximum value of about four to fivetimes the filament diameter (4˜5×d). This maximum value is dictated forreason of constructional stability.

Both the first crimp pitch P_(c1) and the second crimp pitch P_(c2) maybe varied independently of each other. So P_(c1) may be equal to P_(c2)or may be different from P_(c2). The more P_(c1) differs from P_(c2),the more easy it is to prevent the first crimp from tilting. Bothpitches may vary between a minimum value which is about five times thefilament diameter d (5×d), and a maximum value of about fifty times thefilament diameter d (50×d). It is, however, to be preferred, that intwisted structures at least one, and most preferably both, of the crimppitches is smaller than the twist pitch of the steel filament in thetwisted structure.

Having regard to the above parameters which may be chosen quite freely,i.e. independent of each other, a large variety of wave forms can beobtained.

A first example is that by choosing A₁ equal to A₂ and P_(c1) equal toP_(c2) and by shifting the second crimp with a quarter of a pitch inrespect of the first crimp, a spatial helix form can be obtained or atleast be approximated without the need for driven rotatory preformingpins.

A second example is that by choosing A₁ substantially greater than A₂ anoval or elliptical transversal cross-section is obtained. The steelfilament 10 provided with the first and second crimp can be used assingle steel filament, e.g. to reinforce the breaker ply of a rubbertyre.

The steel filament 10 provided with the first and second crimp can alsobe used in a more complex steel structure, next to other reinforcingelements. This more complex structure can be an untwisted structure or atwisted structure where two or more steel filaments are twisted witheach other.

Conveniently, a twisted structure can be made in two ways which differbasically from each other.

A first way, sometimes referred to as “cabling”, is carried out by meansof a rotary tubular strander. According to this technique, theindividual steel filaments are not rotated around their own axis. Thisbe derived, e.g. by means of a microscope, from non-rotating drawinglines (drawing lines are unavoidable imperfections caused by the finalcold drawing steps in the relatively soft brass layer; these drawingsteps conveniently immediately precede the twisting step).

A second way, sometimes referred to as “bunching”, is carried out bymeans of a double-twister. According to this technique, the individualsteel filaments are rotated around their own axes. This can be derivedfrom rotating drawing lines. Both ways are known as such in the art.

The inventors have developed following procedure to detect whether ornot a steel filament has been provided with a first crimp and a secondcrimp according to the invention. If the structure has been “cabled”,then the filaments are simply disentangled from the steel structure. Ifit is possible, e.g. by means of a microscope to discover on adisentangled filament

a) two crimps lying in different planes, e.g. by rotating the steelfilament; or

b) two different crimp pitches; or

c) two different crimp amplitudes; or

d) any combination of a), b) or c), then this filament has been providedwith a first crimp and a second crimp according to the invention.

If the structure has been “bunched”, then the structure must beuntwisted to such a degree that no applied torsions and no residualtorsions are present any more. After this untwisting, one can proceed asfor the “cabled” structures.

Of course other detection techniques can be developed. For example, aKEYENCE LS laser scan, such as disclosed in WO-A-95116816, can be madeof steel filaments and a Fourrier analysis can be applied. In case of“bunching” the bunching frequency can be filtered out and two crimpfrequencies and their higher harmonics will remain.

FIGS. 4 to 8 all show transversal cross-sections of twisted steelstructures which comprise one or more steel filament provided with afirst crimp and a second crimp. The steel filaments provided with afirst crimp and a second crimp are all referred to by reference number10 and their cross-section is to cross-hatched, whereas thecross-section of other steel filaments, if any, is only hatchedobliquely in one direction.

FIG. 4 shows the cross-section of a 4×0.28 steel cord 24. Two filaments10 are provided with a first and a second crimp in order to allow rubberpenetration into the steel cord 24 even if the steel cord 24 is putunder a certain tensile load. Two filaments 26 are not provided withthese crimps.

The number of filaments which should be provided with the crimps inorder to promote rubber penetration depends upon the total number offilaments in the steel cord. The higher total number of filaments, thehigher the number of filaments to be provided with the crimps.

The number of filaments which should be provided with the crimps, inorder to promote rubber penetration also depends upon the amplitude andthe pitch of the crimps. Generally, the higher the amplitudes and thesmaller the pitches, the more rubber is able to penetrate and thesmaller the number of filaments provided with the crimps.

FIG. 5 shows a cross-section of a 5×0.38 steel cord 28 where all thefive steel filaments 10 are provided with the two crimps in order toobtain a high elongation at break (see results in table 5 hereafter).

FIG. 6 shows a cross-section of a 1+6 steel cord 30 where the singlecore filaments 10 is provided with the first and the second crimp. Allfilaments 26 of the layer surrounding the core filament are not providedwith those crimps. The first crimp amplitude is much greater than thesecond crimp amplitude of the core filament so that an ovalcross-section of the steel cord can be obtained. In case it is desiredthat this oval shape does not rotate along the length of the steel cord,the core filament must not rotate in the final twisted steel cord. Thisis no problem if the cabling technique is applied. If the bunchingtechnique is applied, use can be made of the teaching of EP-A1-0 676 500to compensate for the rotating of the core filament around its own axis.

As an alternative of this embodiment, the core filament is provided withthe first and the second crimp and the six filaments of the layer areprovided with a polygonal form as has been disclosed in WO-A-95/16816.

As another alternative of this embodiment, both the core filament andthe six outer filaments are provided with the first and the secondcrimp.

FIG. 7 shows a cross-section of a 12×0.20 compact cord where the threecentral filaments 10 are provided with a first and a second crimp. Thenine outer filaments 26 are not provided with these crimps. Dependentupon the crimp amplitudes and crimp pitches applied, the wavy form ofthe central filaments 10 can give the necessary rubber penetration tothe compact steel cord without necessitating the use of centralfilaments which are thicker than the outer filaments. In case the rubberpenetration is still not satisfactory, or in case a satisfactory rubberpenetration requires too high a crimp amplitude, the nine outerfilaments 26 may also be provided with a first and a second crimp.

FIG. 8 shows a cross-section of a 4×2×0.35 elongation cord 34 where allcomposing filaments 10 are provided with a first and second crimpaccording to the invention. The twist pitch of the 0.35 filaments in the2×0.35 strand can be increased from 3.5 mm to 6.0 mm and the twist pitchof the four 2×0.35 strands in the 4×2×0.35 cord can be increased from 9mm to 16 mm without decreasing the elongation at fracture.

EXAMPLE 1

A first steel filament with a diameter of 0.28 mm has been provided witha first crimp with first crimp amplitude A₁=0.50 mm and first crimppitch P_(c1)=5.0 mm, and with a second crimp with second crimp amplitudeA₂=0.50 mm and with a second crimp pitch P_(c2)=3.0 mm.

A second steel filament with a diameter of 0.28 mm has been providedwith a first crimp with first crimp amplitude A₁=0.75 mm and first crimppitch P_(c1)=5.0 mm, and with a second crimp with second crimp amplitudeA₂=0.50 mm and with a second crimp pitch P_(c2)=3.0 mm.

The above parameters A₁, A₂, P_(c1) and P_(c2) are all parameters astuned on the crimp wheels. As may be derived hereinafter, the effectiveparameters as measured on the filaments may deviate from the parametersas tuned, e.g. because the second crimp influences the first crimpparameters. A possible downstream twisting of the filaments into a steelcord may also influence both the crimp amplitudes and the crimp pitches.Downstream operations usually decrease the crimp amplitudes and increasethe crimp pitches.

Both filaments have been compared with a non-crimped steel filament of0.28 mm diameter as reference.

TABLE 1 0.28 filament Ref. 1st 2nd filament filament filament Breakingload F_(m) (N) 157 145 145 Elongation at break (%) 1.5 4.1 6.0 A₁measured on filament (mm) 0.280 0.455 0.796 P_(c1) measured on filament(mm) 0.000 5.319 5.265 A₂ measured on filament (mm) 0.280 0.420 0.467P_(c2) measured on filament (mm) 0.000 3.126 3.119

Both the first filament and the second filament have been used to makefour invention embodiments of a 4×0.28 steel cord with twisting pitchP=16.0 mm.

Embodiment no. 1 comprises one filament crimped as the first filamenthereabove and three non-crimped filaments.

Embodiment no. 2 comprises two filaments crimped as the first filamenthereabove and two non-crimped filaments.

Embodiment no. 3 comprises one filament crimped as the second filamenthereabove and three non-crimped filaments.

Embodiment no. 4 comprises two filaments crimped as the second filamenthereabove and two non-crimped filaments.

The four embodiments are compared with a reference 4×28 open cord with atwisting pitch of 16.0 mm.

TABLE 2 4 × 0.28 cord Embodiment no. 1 2 3 4 Ref. breaking load F_(m)(N) 616 585 597 548 660 tensile strength R_(m) 2526 2397 2444 2239 2657(MPa) E-modulus (MPa) 178586 162527 167564 144043 permanent elongationat 1.12 0.88 0.93 0.72 max. load (%) total elongation at break 2.5 2.42.4 2.3 (%) yield strength at 0.2% 2074 1976 2074 1902 permanentelongation (MPa) PLE cord at 50 N (%) 0.133 0.174 0.177 0.223 0.400 PLEof crimped filament 0.840 0.874 1.179 1.210 — at 50 N (%) PLE ofnon-crimped 0.590 0.584 0.602 0.548 filament at 50 N (%) Rubberpenetration % pressure drop 0 0 0 0 0-20 appearance rating 55 56 50 58inside cord (%)

The “part load elongation or PLE of a steel element (Whether steel cordor steel filament) at 50 Newton” is defined as the increase in length ofthe steel element, Which results from subjecting the steel element to adefined force of 50 Newton—and is expressed as a percentage of theinitial length of the steel element measured under a defined pre-tension(of e.g. 2.5 Newton).

The rubber penetration has been measured in two ways. A first way is theconvenient and well-known pressure drop test. A second way determinesthe so-called appearance rating and is measured here on the corefilament in the following way. The twisted cord is embedded in rubberunder conditions comparable to manufacturing conditions. Thereafter theindividual steel filaments are unraveled and the appearance rating isthe surface area of a particular steel filament covered with rubbercompared with the total surface area of that particular steel filament.in this measurement of the appearance rating, the numerical results arevery dependent upon the type of rubber used.

EXAMPLE 2

A first high-tensile steel filament with a diameter of 0.38 mm has beenprovided with a first crimp with first crimp amplitude A₁=1.0 mm andfirst crimp pitch P_(c1)=5.2 mm, and with a second crimp with secondcrimp amplitude A₂=0.75 mm and with a second crimp pitch P_(c2)=3.2 mm.

A second high-tensile steel filament with a diameter of 0.38 mm has beenprovided with a first crimp with first crimp amplitude A₁=1.0 mm andfirst crimp pitch P_(c1)=5.2 mm, and with a second crimp with secondcrimp amplitude A₂=0.50 mm and with a second crimp pitch P_(c2)=3.2 mm.

A third high-tensile steel filament with a diameter of 0.38 mm has beenprovided with a first crimp with first crimp amplitude A₁=0.75 mm andfirst crimp pitch P_(c1)=5.2 mm, and with a second crimp with secondcrimp amplitude A₂=0.75 mm and with a second crimp pitch p_(c2)=3.2 mm.

The above parameters A₁, A₂, P_(c1), and P_(c2) are all parameters astuned on the crimp wheels. As may be derived hereinafter from Table 3,the effective parameters as measured on the filaments may deviate fromthe parameters as tuned, e.g. because the second crimp influences thefirst crimp parameters. A possible downstream twisting of the filamentsinto a steel cord may also influence both the crimp amplitudes and thecrimp

TABLE 3 0.38 mm filament Ref. 1st 2nd 3rd filament filament filamentfilament Breaking load 312 267 279 271 F_(m) (N) Elongation at 1.5 10.116.54 7.10 break (%) E-modulus 200000 44830 54777 80028 (MPa) A₁ measuredon 0.38 0.846 0.918 0.634 filament (mm) P_(c1) measured 0 5.143 5.1705.198 on filament (mm) A₂ measured on 0.38 0.684 0.497 0.621 filament(mm) P_(c2) measured 0 3.150 3.141 3.047 on filament (mm)

With the above-mentioned three types of high-tensile filaments nine5×0.38 invention cords with twisting pitch 14.0 mm have been madeaccording to table 4 hereunder where all of the steel filaments havebeen provided with both the first and the second crimp.

TABLE 4 filament composition of cords Invention cord no. Filament no. noadditional much additional some additional ↓ preforming preformingpreforming 1 1 4 7 2 2 5 8 3 3 6 9

Table 5 hereunder compares the results of these nine invention cordswith a reference 5×0.38 high-tensile open steel cord with twisting pitch12.0 mm.

TABLE 5 5 × 0.38 cord Ref. Invention cord no. cord 1 2 3 4 5 6 7 8 9linear density 4.543 4.498 4.493 4.484 4.471 4.467 4.495 4.462 4.482(g/m) max. diameter 1.495 1.239 1.236 1.412 1.402 1.296 1.334 1.2771.310 (mm) PLE cord at 0.261 0.204 0.153 0.378 0.341 0.337 0.260 0.2280.269 50N (%) PLE filament at 1.568 1.289 1.194 1.484 1.363 1.327 1.4181.186 1.331 50N (%) breaking load 1540 1308 1373 1331 1309 1408 13161305 1396 1347 F_(m) (N) tensile strength 2686 2262 2400 2329 2295 24752316 2281 2459 2362 R_(m) (MPa) E-modulus 193000 70773 98615 107267105908 135309 138711 99710 155702 122695 (MPa) permanent 1.7 3.95 1.832.89 2.52 1.58 1.78 2.74 1.29 2.32 elongation at max. load (%)elongation at 3.8 7.38 4.43 5.15 5.11 3.70 3.73 5.45 3.04 4.95 break (%)yield strength 93 48 66 59 60 73 69 56 78 62 at 0.2% permanentelongation (MPa) rubber 0 0 0 0 0 0 0 0 0 0 penetration (pressure drop -%) Breaking load 1667 1399 1508 1461 1376 1467 1356 1381 1506 1446 F_(m)embedded in rubber (N) Total 2.09 6.39 3.33 4.3 3.21 1.93 1.98 3.8 1.953.59 elongation at break embedded in rubber (%) compression 19000 2632635590 65471 54423 65707 71853 54389 75526 64140 modulus (MPa)deformation at 4.23 1.60 1.45 0.79 1.21 1.01 0.87 1.11 0.72 0.94instability w_(k) (%)

Following explanation can be given with respect to the values derivedfrom a compression test. Due to their high length-to-diameter ratiosteel cords as such have no resistance to compression. Once embedded inrubber, however, a steel cord can build up a considerable compressionresistance. A cylinder test has been developed, which providesinformation on the compression properties of rubber-embedded steelcords. A rubber cylinder with a diameter of 30 mm and a height of 48.25mm is reinforced exactly in the center with a test steel cord. By meansof a precision mold and by tensioning the steel cord during curing, thecord is kept straight and exactly in the axis of the cylinder. Thecompression test records a force versus deformation diagram. W_(k) isthe deformation at instability or at the buckling point. Further detailsabout the compression test may be read from L. BOURGOIS, Survey ofMechanical Properties of Steel Cord and Related Test Methods, SpecialTechnical Publication 694, ASTM, 1980. A steel cord for protection pliesis said to have a good compression behavior if W_(k) exceeds 3%.

The values of the E-modulus or modulus of elasticity mentioned in Table5 are average values. When performing a tensile test and recording aload-elongation curve, however, two different E-moduli can be observed.The two different E-moduli are a consequence of two crimps withdifferent crimp pitches. In a tensile test, the crimp with the smallercrimp pitch leads to the elongation at the smaller loads, while thecrimp with the greater crimp pitch only gives effective elongation athigher loads. This is shown in FIG. 9, which gives the load-elongationcurve of invention cord no. 1 of Table 5 hereabove. Two clearly distinctE-moduli are shown by means of point-dash lines.

EXAMPLE 3

A number of 0.22 mm filaments have been provided with the first andsecond crimp. Table 6 hereunder compares a number of their propertieseach time with a straight 0.22 mm filament as reference.

TABLE 6 0.22 mm filament 1st filament 2nd filament 3rd filament 2x 2x 2xRef crimp Ref crimp Ref crimp Breaking load 117 103 118 103 117 101 (N)tensile strength 3080 2711 3115 2705 3067 2669 (MPa) yield strength 26172053 2525 2022 2539 1985 R_(p) at 0.05% elongat. (MPa) R_(p) at 0.1%2839 2202 2851 2133 2798 2109 elongation (MPa) R_(p) at 0.2% 3005 23503029 2275 2975 2259 elongation (MPa) elongation at 0.38 0.95 0.45 0.960.45 0.94 max. load (%) tot. elongation 1.88 3.48 1.9 4.3 1.91 3.84 atfracture (%) A_(1 (mm)) 0 0.47 0 0.49 0 0.47 P_(c1) (mm) 5.20 5.16 5.31A_(2 (mm)) 0 0.34 0 0.40 0 0.39 P_(c2) (mm) 3.04 3.04 3.12

EXAMPLE 4

A 12×0.38 compact cord has been manufactured with all twelve filamentsprovided with the double crimp. The cord can be used as weft element ina woven structure adapted to reinforce conveyor belts. Four embodimentsof the 12×0.38 compact cord (CC) are compared with a conventional4×7×0.25 high-elongation (HE) cord. The differences between the fourembodiments of the 12×0.38 compact cord are as follows:

Number 1: low winding tension, low rotation speed of buncher;

Number 2: high winding tension, low rotation speed of buncher;

Number 3:low winding tension, high rotation speed of buncher

Number 4: high winding tension, high rotation speed of buncher.

TABLE 7 12 × 0.38 compact cord 12 × 0.38 - invention 1 2 3 4 4 × 7 ×0.25 Lay length (mm) 18 S 18 S 18 S 18 S 5/10 SS Rubber penetration 0 00 0 100 Pressure drop (%) Optical diameter D_(min) 2.105 1.940 1.8881.910 1.879 D_(max) 2.439 2.285 2.334 2.399 2.131 Linear density (g/m)11.184 11.154 11.14 11.181 11.77 Cross-sectional 1.42 1.42 1.42 1.411.50 surface (mm²) Tensile test on not embedded cord Breaking load (N)2856.7 2787.7 2840.7 2727.3 3149.7 Tensile strength 2008 1965 2004 19282103 (MPa) E-modulus (MPa) 44371 46998 49113 48907 101858 yield strengthat 0.01 630 708 661 688 1156 % elongation (MPa) Yield strength at 47 5250 54 78 0.2% elongation (%) Elongation at 3.35 2.94 3.29 2.79 2.45maximum load (%) Total elongation at 7.88 7.12 7.38 6.73 4.66 fracture(%) Tensile test on cord embedded in rubber Breaking load (N) 3023.472750.4 2905.07 2747.73 3345.07 Tensile strength 2122 1936 2047 1940 2231(MPa) E-modulus (MPa) 65373 70380 71446 71508 117420 yield strength at0.2 1153 1183 1200 1219 2044 % elongation (MPa) yield strength at 0.2 5461 59 63 92 % elongation (%) Elongation at 3.46 2.49 2.93 2.32 0.94maximum load (%) Total elongation at 6.71 5.24 5.8 5.05 2.91 fracture(%)

EXAMPLE 5

A 4×0.30 cord and a 5×0.30 cord have been manufactured starting from0.30 mm filaments which have all been provided with the double crimp.The amplitude of the first crimp as tuned on the toothed wheel was 0.70mm and the pitch of the first crimp as tuned on the toothed wheel was5.2 mm. The amplitude of the second crimp as tuned on the toothed wheelwas 0.55 mm and the pitch of the second crimp as tuned on the toothedwheel was 3.2 mm. Table 8 hereunder summarizes the measured properties.

TABLE 8 4 × 0.30 and 5 × 0.30 4 × 0.30 5 × 0.30 invention invention laylength cord (mm) 12.5 12.5 linear density (g/m) 2.255 2.824 Surface oftransversal cross- 0.29 0.36 section (mm²) Breaking load (N) 787 972.3Tensile strength (MPa) 2743 2706 E-modulus 115013 113222 Yield strengthat 0.01% 1099 1091 elongation (MPa) Yield strength at 0.2% 1683 1707elongation (MPa) Yield strength at 0.2% 61 63 elongation (%) Elongationat maximum load (%) 2.07 1.98 Total elongation at fracture (%) 4.46 4.37

In addition to the above-mentioned characteristics and properties, asteel cord according to the present invention has following featureswhich make it able for the reinforcement of elastomers such as rubber:

the filament diameters range from 0.04 mm to 1.1 mm, more specificallyfrom 0.15 mm to 0.60 mm, e.g. from 0.20 mm to 0.45 mm;

the steel composition generally comprises a minimum carbon content of0.60% (e.g. at least 0.80%, with a maximum of 1.1%), a manganese contentranging from 0.20 to 0.90% and a silicon content ranging from 0.10 to0.90%; the sulphur and phosphorous contents are each preferably keptbelow 0.03%; additional elements such as chromium (up to 0.2 a 0.4%),boron, cobalt, nickel, vanadium may be added to the composition;

the filaments are conveniently covered with a corrosion resistantcoating such as zinc or with a coating that promotes the adhesion to therubber such as brass, or a so-called ternary brass such ascopper-zinc-nickel (e.g. 64%/35.5%/0.5%) and copper-zinc-cobalt (e.g.64%/35.7%/0.3%), or a copper-free adhesion layer such as zinc-cobalt orzinc-nickel.

The invention is suitable for all common and available final tensilestrengths from 2150 MPa to about 3000 MPa and more.

What is claimed is:
 1. A steel structure adapted for the reinforcementof elastomers, said steel structure comprising one or more steelfilaments, said steel filaments having a diameter ranging from 0.04 mmto 1.10 mm, said steel filaments having a steel composition comprising aminimum carbon content of 0.60%, a manganese content ranging from 0.20%to 0.90%, and a silicon content ranging from 0.10% to 0.90%, said steelfilaments being covered with a corrosion resistant coating or with acoating that promotes the adhesion to rubber, wherein at least one ofsaid steel elements is provided with a first crimp and a second crimp,the first crimp lying in a plane that is substantially different fromthe plane of the second crimp.
 2. A steel structure according to claim 1wherein said steel filament has a round cross-section.
 3. A steelstructure according to claim 2 wherein said first crimp has a firstcrimp amplitude and said second crimp has a second crimp amplitude, saidfirst crimp amplitude and said second crimp amplitude varying between aminimum value of 1.05×d and between a maximum value of 5×d, d being thediameter of the steel filament.
 4. A steel structure according to claim1 wherein said structure comprises between two and five steel filaments.5. A steel structure according to claim 4 wherein said structureconsists essentially of two to five steel filaments, one to four steelfilaments of which are provided with said first and second crimp inorder to allow rubber penetration.
 6. A steel structure according toclaim 4 wherein said structure consists essentially of two to five steelfilaments and wherein all these filaments are provided with said firstand second crimp in order to obtain an increased elongation at fracture.7. A steel structure according to claim 1 wherein said filaments aretwisted with each other with a twist pitch.
 8. A steel structureaccording to claim 7 wherein a first crimp pitch is smaller than saidtwist pitch.
 9. A steel structure according to claim 7 wherein a secondcrimp pitch is smaller than said twist pitch.
 10. A steel structureaccording to claim 1 wherein said first crimp has a first crimp pitchand said second crimp has a second crimp pitch, said first crimp pitchbeing different from said second crimp pitch.
 11. A steel structureaccording to claim 10 wherein said structure exhibits two substantiallydifferent moduli of elasticity during a tensile test.
 12. A steelstructure according to claim 1 wherein a first crimp amplitude isdifferent from a second crimp amplitude.